1. Field of the Invention
This invention relates to a low residue soldering flux and to a method of using it. More particularly, it relates to a water-based flux that contains a very special mixture of three dibasic acids that not only provides effective fluxing action at low temperatures but also provides a resultant soldered product that is substantially ionic residue-free.
2. Technical Field
A soldering flux is used to aid fusion of the parts being joined by reacting with and dissolving their surface oxides or impurities while coating the surfaces against oxidation, during soldering, at a temperature above the solder melting point.
Organic fluxes are typically based on water-insoluble rosin or water-soluble organic acid. Activated rosin fluxes are used in soldering electrical connections on printed circuit boards. Wave soldering is used for mass production circuit board soldering as by applying the flux, preheating the board, applying the solder (e.g., via a molten solder standing wave), cooling the board, and cleaning it to remove flux residue.
Such flux residue is mainly constituted of ionic (e.g., acidic or basic) substances, and is corrosive, or can hydrolyze to corrosive constituents in the presence of moisture (e.g., air moisture). This can lead to short circuits, noise generation, etc., in use of the circuit board product. These adverse results are effectively avoided by subjecting the soldered board to a cleaning step to remove the ionic substances.
U.S. Pat. No. 5,004,509 discloses a low residue soldering flux that has overcome many of the disadvantages faced by prior art fluxes. The flux of that patent contains a Three-acid mixture of succinic, glutaric and adipic acids. The Three-acid mixture is sometimes referred to herein as TAM. The TAM of U.S. Pat. No. 5,004,509 is employed either with or without a rosin component, and is dissolved in a suitable organic solvent, namely a volatilizable organic solvent. U.S. Pat. No. 5,004,509 discloses that water can only be used in minor amounts in a flux having both rosin and the Three-acid mixture. Further, in the rosin free flux of U.S. Pat. No. 5,004,509, water may be employed but only in a slight amount, i.e., at most about 1% of the total vehicle. The use of only minor amounts of water in the flux is specifically emphasized at Col. 7, lines 37-40 of U.S. Pat. No. 5,004,509, in order "to inhibit splattering and deposition of residue at the soldering site, and possibly solder ball formation, that adversely affect product quality." Moreover, where water is employed in a flux of U.S. Pat. No. 5,004,509, which contains an aliphatic carboxylic acid ester (as a wetting agent), the flux also desirably includes an hydrolysis inhibiting agent to control the extent of hydrolysis of the ester that occurs in the presence of water. This was another reason expressed for keeping water content in the flux low.
A number of background patents are discussed in U.S. Pat. No. 5,004,509, including U.S. Pat. Nos. 2,715,084, 2,898,255, 2,904,459, 3,235,414, 3,264,146, 3,837,932 and 4,168,996. Another composition is disclosed in SU 1017460.
The importance of a residue-free soldered connection is discussed in the article entitled: "Europeans find ways to phase-out CFCs," Electronic Packaging & Production, Vol. 29, No. 1, January 1989, pp. 26 and 28, Linda Smith-Vargo, Associate Editor. The article notes that fluxes used in printed circuit board applications leave residues that must be cleaned, typically by an organic liquid based on fully halogenated chlorofluorocarbons or CFCs, but that in view of the need to reduce CFC emissions, due to environmental constraints, soldering procedures are needed that avoid CFCs yet produce a residue-free non-corrosive product.
It is desirable to have a flux for soldering metal connections in electrical applications such as printed circuit boards that does not produce a corrosion-causing and/or hydrolyzable ionic residue that requires a cleaning step for its removal.